The invention is in the field of emissive materials for display applications. More specifically, it pertains to the use of organic luminescent materials in light emitting devices.
Organic light emitting devices (OLED) are an attractive alternative to current display technologies such as plasma based displays and vacuum fluorescence because they offer advantages of low cost, likely ease of manufacture, large area, wide viewing angle and open up the possibility of flexible displays. Flexible displays can have host of consumer applications. In a typical organic light emitting device, light is given off when electrons and holes recombine after they are transported into the light emission layer.
Depending upon the properties of the light emissive material making up the light emission layer, devices can emit a given color in the visible range of the electromagnetic spectrum. For example, one can create devices emitting red, green or blue color (RGB). Some of the devices already described in the published literature have used known luminescent compounds as light emissive materials. For example, U.S. Pat. No. 5,674,635 describes the use of Coumarin-6, a known fluorescent dye as emitter material for green light in an OLED.
One of the problems faced in OLEDs for RGB displays, is that the color saturation or purity is not very satisfactory due to a large spectral width of the each color component (Forrest et. al., Laser Focus World, pp. 104, February 1995). Usually, the large spectral width of electroluminescent (EL) spectrum is inherent of the starting luminescent material because EL spectrum generally matches photoluminescence (PL) spectrum. Therefore, choosing, the light emissive material having smaller PL emission spectral width is one approach to solving the color purity problem.
Unfortunately, organic luminescent materials such as coumarins, pyrazoles, pyrazolines etc., possess broad PL spectrum rendering their color saturation less than desirable. Cyanines are a class of organic compounds that possess sharp absorption and emission spectra. More specifically, cyanines that exhibit higher fluorescent quantum efficiencies due to their rigid structures are desirable. U.S. Pat. No. 5,294,870 discusses the use of trimethine and pentamethine cyanine dyes for emitting green and red colors and of other cyanine dyes covered by Weissberger and Taylor, Special Topics of Heterocyclic Chemistry, John Wiley and Sons, New York, 1977, Chapter IIIxe2x80x3 for use in OLED assemblies. However, rigidized dyes structures shown in Chapter VIII are rigidized by carbon atoms and do not mention any other rigidizing atoms. Moreover, the cyanines dyes structures suggested by U.S. Pat. No. 5,294,870 and the references therein contain counterions.
Monomethine cyanines (those with one carbon atom in the chromophoric linkage) based on pyrrole heterocycle and rigidized by boron atom was used as a dopant in HTL and ETL in constructing EL device (Chem. Abs., Vol. 127:364033s) and subsequently structural variations of the same basic chromophore were incorporated in EL devices (Polym. Prep., Vol. 38(1), pp390, 1997 and Polym. Prep. Vol.38(1), pp.339, 1997). However, the use of this boron rigidized pyrrole based monomethine cyanine dye in EL devices reported is limited to generating light in green or red region of visible light spectrum. The dye structure simply does not permit its use in EL device for creating blue light. So there is a need for emitter materials suitable for generating blue color of sufficient color purity in an OLED.
Recently, some rigidized cyanines were patented (U.S. Pat. No. 5,852,191) as new chemical compounds for use in fluorescent labeling, as textile and laser dyes. Unlike the monomethine cyanines suggested by U.S. Pat. No. 5,294,870 these compounds are devoid of counterions so structurally quite distinct from traditional cyanine dyes. These dyes are efficient luminophors (efficiency 0.9), possess good light fastness, sharp absorption and emission spectra. However, the rigidized cyanines described in U.S. Pat. No. 5,852,191 have not been proposed for application as light emissive materials for OLEDs.
In one aspect the invention is directed to the method of using the rigidized cyanines of U.S. Pat. No. 5,852,191 as emissive materials in an organic light emitting device (OLED) assembly. It is further the object of the invention to feature an OLED assembly emitting blue light. It is further the object of the invention to feature an OLED assembly emitting blue light in which the emissive material is deposited as a layer using conventional small molecule deposition technique such as thermal vacuum deposition. It is further an object of the invention to feature OLED assembly in which the emissive material is dispersed as a dopant in the hole transport layer or an electron transport layer. Co-deposition of emissive material with the hole transport material or an electron transport material can be achieved by thermal vacuum deposition. Another way to achieve doping of the emissive material is by dissolution in a suitable solvent along with an electron transport material and a polymeric hole transport material. It is further the object of the invention to feature OLED assembly having emissive material as a layer or as a dopant to emit light covering the 500 nm-700 nm region of the visible spectrum.